As a general definition, separating deices can be broadly divided into sorting devices which separate granular mixtures that contain different materials of different densities and classifying devices which separate particles of a homogeneous granular mixture by size (i.e., mixture comprised of particles having the same density). Conceptually, a sorting device must first classify the non-homogeneous particles of the granular mixture into similarly sized particles usually by screening and then separate the particles by their weight or density differences. This invention relates to classifiers for sorting or separating essentially homogeneous granular mixtures by particle size.
The typical, commercial method for separating or classifying particles in a granular mixture into various sizes is to simply pass the particles through screens through desired grid sizes. Because the grids plug, any number of devices or techniques are employed to unplug or minimize clogging of the screen grid. For example, air jets can be periodically or continuously blown through the screen grids or the screens are continuously vibrated or scraped or brushed. Also, special structures can be constructed so that the screens or the particles can be moved relative to one another to prevent screen plugging such as shown in U.S. Pat. No. 5,429,248 to Le Gigan et al. issued Jul. 4, 1995 in which tubular screens rotate while the granular mixture falls thereon. All such classifying devices have the potential for failure if the screens are blocked. Further, such classifying devices invariably have moving parts or mechanisms which are prone to or suspect of failure in the harsh environment of a granular mixture that inevitably produces or includes dust particles and fines.
The prior art has long recognized the plugging problems associated with screen classifiers discussed above and has developed classifiers which utilize air pressure and flow to separate particles by size. Examples of such devices can be found in U.S. Pat. No. 3,799,339 to Breitholtz et al.; issued Mar. 26, 1974; U.S. Pat. No. 4,519,896 to Vickery issued May 28, 1985; U.S. Pat. No. 4,950,388 to Stafford issued Aug. 21, 1990; U.S. Pat. No. 5,032,256 to Vickery issued Jul. 16, 1991; U.S. Pat. No. 5,732,829 to Smith issued Mar. 31, 1998 and U.S. Pat. No. 5,871,103 to Durst issued Feb. 16, 1999. Essentially, in these devices the granular mixture falls vertically by gravity across a horizontal stream or flow of air. The particles become entrained in the air stream and are carried horizontally some distance. The entrained large particles weigh more than the smaller particles, accelerate more slowing in the air stream and drop out of the air stream (by gravity) quicker and at a shorter horizontal distance from the point where the granular mixture was initially contacted by the air stream than the smaller particles. By positioning hoppers or receptacles at set horizontal distances, particles of different sizes can be collected. Conceptually, screens and moving parts are eliminated although a number of the patents cited use systems which employ moving components. However, such horizontal air tunnel classifiers are difficult to control. For example, the overall variation in the particle size of the granular mixture can be such that a predominance of larger particles in one granular mixture black can distort the drop point when compared to another granular mixture batch having a predominance of smaller particle sizes. Additionally, many of the cited patents discuss laminar flow and include provisions or adjusting mechanisms in the air feed to assure laminar flow. The cited patents use arrangements which push or blow the air into the granular mixture to entrain or suspend the particles. “Pushed” air streams at low, laminar flow velocities are not stable and when the low velocities have to be slightly varied to allow particles within a set discriminatory size range to drop out of the air streams at set distances, controllability issues are present.
Variations of the horizontal tunnel air classifiers discussed above exist in the patent art. In U.S. Pat. No. 4,759,840 to McIntyre et al. issued Jul. 26, 1988 a recirculating, filtered fan arrangement is shown to develop turbulent and laminar air streams which sequentially impact the granular mixture. In U.S. Pat. No. 3,612,271 to Behling issued Oct. 12, 1971 a push-pull fan arrangement is disclosed to suck lighter capsules from completely filled capsules. In U.S. Pat. No. 3,044,619 to Knolle issued Jul. 17, 1962 the particles are separated by a screen for size and then generally horizontally flow past air streams where they are blown upward. The upward air streams impart a trajectory to the particles which fall into horizontally placed receptacles. For the specific applications to which such patents are directed, there may be improved operational results compared to the conventional horizontal air tunnel arrangements discussed above although control and stability remain a concern.